Extending the depth of focus of an imaging system is required for various applications, including medical application, such as ophthalmic applications. Various techniques have been developed to extend the depth of focus of an imaging system.
For example, the earlier technique developed by the inventors of the present application provides an extended depth of focus of an imaging lens unit by applying a phase coding to the effective aperture of the imaging lens. The features of this technique are described in the following patents and published patent applications: U.S. Pat. No. 7,365,917; U.S. Pat. No. 7,061,693; US 2009/074239; US 2009/116096; U.S. Pat. No. 7,646,549, all assigned to the assignee of the present application. Some other techniques for extending depth of focus for imaging and other optical applications are described for example in the following patents: U.S. Pat. No. 6,554,424, U.S. Pat. No. 6,097,856 and U.S. Pat. No. 6,069,738.
Yet another technique is disclosed for example, U.S. Pat. No. 7,224,540, which describes an imaging system that has a wavelength dependent focal shift caused by longitudinal chromatic aberration in a lens assembly that provides extended depth of field imaging due to focal shift and increased resolution due to reduced lens system magnification. In use, multiple wavelengths of quasi-monochromatic illumination, from different wavelength LEDs or the like, illuminate the target, either sequentially, or in parallel in conjunction with an imager with wavelength selective (colored) filters. Images are captured with different wavelengths of illumination that have different focus positions, either sequentially or by processing the color planes of a color imager separately. Extended depth of field, plus high resolution is achieved. Additionally, information about the range to the target can be determined by analyzing the degree of focus of the various colored images.